Sundials&

Starfinders

SundialsSundials• Sundials come in variety of forms but all

sundials measure “Solar Time”

SundialsSundials• Basic Parts:

» Hours markers» Gnomon – A device that cast a shadow on

the hour markers.

• Basic Premise: If we divide the day into 24 hours and the Earth rotates about 360° each day then,

Each hour on a sundial is 15° wide.• But…….

SundialsSundials• The Gnomon must be aligned with the

Earth’s rotation axis for the sundial to work properly.

SundialsSundials• There is an alternate version of a sundial

called an “Analematic Sundial”

• This style of sundial uses a vertical gnomon.

• However, the gnomon must be movable and calendar positions must be determined for each day’s gnomon location.

SundialsSundials• Analemmatic Sundial

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There’s still a problem

SundialsSundials• The Solar Day is not always 24

hours long due to our changing orbital speed.

• The Solar Day can be 15 minutes longer or shorter than the clock day depending on the time of year.

SundialsSundials

SundialsSundials• The Result:

– If we want the sundial to read the same time as the clock, we need to add or subtract a correction factor

– This factor may also include Daylight Savings Time.

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To read the sundial:

1.Orient sundial so that the gnomon points north.

2. Read the hour markers to determine solar time.

3. Add or subtract appropriate correction factor.

SundialsSundials• Try some examples

» On October 13th, what time is it if the sundial reads 2:30 pm?

» On January 25th, what time is it if the sundial reads 10:00 am?

» When is there no correction to the sundial?

» When is Daylight Saving Time the only correction?

SundialsSundials• Another use for the Sundial is finding

directions.

• To find your directions:– Use a clock to find the time.– Perform a reverse correction to determine the

correct sundial reading at that time.– Turn sundial until it reads the corrected time.– You are now facing north.

StarfindersStarfinders• In ancient times, starfinders were called

astrolabes.

• They were typically made of brass or wood.

• Astrolabs rarely had constellation names, stars only.

StarfindersStarfinders• Identifying stars or constellations:

1. Line up the time of night with the current date.

2. Hold the direction your are facing at the bottom.

3. The constellations (or stars) will be directly in front of you .

StarfindersStarfinders• Rising or setting times:

1. Place the star or constellation on the eastern or western horizon of the starfinder.

2. Read the time for that date.

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• Finding the time:

1. Rotate starfinder until it matches the sky. Pay special attention to the Dippers and Cassiopeia (looks like a “W”).

2. Read the time next to your current date.

StarfindersStarfinders• Which constellation is just above the

eastern horizon at 10:00 pm on the 14th of October?

• Which constellation is just above the southern horizon at midnight on January 15th?

StarfindersStarfinders

Starfinders• What time does time does Sirrius rise

tonight?

• What time does the backward questions mark of Leo rise tonight?

• What time does Aquila set tonight?